1. Field of the Invention
The present invention relates to a laser-power control apparatus employed in an optical recording device. More particularly, a power-control device for an optical disc recording system that controls the optical power stably during high-speed optical information recording even if using a relative low-speed front photodiode is disclosed.
2. Description of the Prior Art
A laser diode is usually used as a light source in a conventional optical recording apparatus. It is well known that the optical power output of a laser diode varies greatly with changes in environmental temperature. Accordingly, it is necessary to compensate for unwanted temperature-induced power fluctuations in the operation of laser diodes. A feedback control device for stabilizing the output power of a laser diode is called an automatic power control (APC) and is generally utilized in a conventional optical recording apparatus.
In order to record information pits into the optical recording medium, the optical output of the laser diode is modulated into recording pulses with different power levels and periods. This is referred to as the write strategy. FIG. 1 shows some common write strategies for present optical disc formats. To obtain a good recording quality, precise power control is necessary. A key to the success of the APC is a correct measurement of the optical output power of the laser diode.
FIG. 2 is a block diagram illustrating a prior art APC structure 10. In this figure, a laser diode LD radiates laser light onto an optical disc (not shown). The light radiated by the LD is received by a front photodiode FPD. The output of the FPD is converted into a voltage signal, front photodiode output (FPDO) signal, through a current-to-voltage conversion unit 20. The FPDO voltages corresponding to bias (read) power, erase power, and write power during reproduction and recording operations are respectively measured by a power level acquisition unit 30. The measured voltages are outputted by the power level acquisition unit 30 as Vb,m, Ve,m, and Vw,m. Feedback controller units 40, in accordance with the individual differences between the reference voltages Vb,r, Ve,r, and Vw,r set by a reference power level setting unit 50 and the measured voltages Vb,m, Ve,m, and Vw,m, output the required bias level, erase level, and write level currents into an LD driving unit 60 for producing desired recording pulses into the LD for recording information pits into the optical disc.
To correctly measure the output power of a laser diode, a sample and hold circuit is usually employed in the power level acquisition unit 30 of a conventional optical recording apparatus. Various sampling signals are issued according to the information data to be recorded and the corresponding write strategy. Then the voltage levels of the FPDO during the bias period, erase period, and write period are sampled and held for feedback control. Precise power control is thus obtained despite fluctuations in the temperature of the LD. Nevertheless, it is implicitly assumed, in the sample and hold scheme, that the response speed of the FPD is approximately the same as the modulation speed of the recording pulse of the LD. In other words, the FPDO must follow tightly the changes of each recording pulse of the LD so that a sampling device can correctly sample the power levels of the LD from the FPDO. For example, as shown in FIG. 3, the time duration of the erase period in a rewritable optical disc format is generally long enough to allow the FPDO to closely approximate the recording pulse of the LD and provide a qualified sampling area for APC.
In a high-speed and/or high-density optical recording application, the response speed of the FPD is likely to be slower than the modulation speed of the recording pulse of the LD. This results in the FPDO having only a short time period in steady state. This problem is illustrated in FIG. 4 where even though the FPDO response reaches steady state near the end of an erase period, a medium-speed sampling device may fail to correctly sample the desired FPDO value.
Obviously, implementation of a high-response-speed sample and hold circuit is expensive. In certain recording formats such as those using blue laser diodes, the recording pulse widths are so short that correctly sampling the FPDO is impossible for present hardware implementation technology under the constraint of reasonable costs. Additionally, as the recording pulses get shorter and shorter, it is very likely that the response speed of the FPD is much slower than the modulation speed of the recording pulse of the LD. As shown in FIG. 5, the FPDO cannot correctly reflect the optical power output of the LD. In this situation, the real output power cannot be measured correctly even with the use of a perfect sample and hold circuit regardless of cost.
The sampling problems in obtaining the write and bias power levels get worse for those optical disc formats with a multiple pulse train write strategies shown in FIG. 1 because the time duration in a modulated multiple pulse is several times less than during a write/erase period. The FPDO will fluctuate, as shown in FIG. 5, and no sampling device can provide correct optical power measurement. In addition, some optical disc formats like blu-ray disc can also adopt a multi-pulse write strategy in the erase period, as shown in FIG. 1. There may be no available sampling areas in the FPDO for the power level acquisitions of the erase, write, and bias periods.
One method employed in the power level acquisition unit 30 is to use a peak (or bottom) envelope detection device, which continuously tracks the peaks (or bottoms) of the FPDO for feedback control. Chuang, herein incorporated by reference, discusses such a device in U.S. Patent Application Publication US 2002/0141313. Here, peak envelope signals outputted from the envelope detection devices are fed to standard sample and hold circuits, which in turn, output to the respective feedback control units. However, to reliably detect and reflect peaks (or bottoms) of the FPDO, the discharge time constant of a peak (or bottom) envelope detection device cannot be too large compared to that of the recording pulses of the LD. If the discharge time constant is too large, the peak (or bottom) envelope detection device may not correctly follow the FPDO. If the discharge time constant is too small, output from the peak (or bottom) envelope detection device may incur small dropouts in the detected peak (or bottom) envelope in spite of the same amplitude for each peak (or bottom) in the FPDO shown in FIG. 6.
Additionally, when the response speed of the FPD is much slower than the modulation speed of the recording pulse of the LD, the FPDO cannot achieve steady state within a recording pulse, and hence the output signal of a peak (or bottom) envelope detection device will also follow the variation due to write strategy, as shown in FIG. 7. Since a peak (or bottom) envelope detection device will track the local maximum peaks existing in the inputted pulses train, the power measured by a peak (or bottom) envelope detection device will continuously change because of the write strategy when an FPD with low response speed is used. Since the acquired power deviations result from the write strategy and temperature drifts simultaneously, the feedback controller unit will perform wrong power adjustments because of erroneously sensed FPDO variations resulting from write strategy. Consequently, it will be difficult to stably compensate real power fluctuation resulting from the effect of temperature.